In making electro-dynamic, that is moving coil, vibration transducers for bending wave panel speakers, current thinking on voice coil size and mass tends towards the use of small diameter and low mass voice coil systems, typically of the size of tweeter coils of conventional pistonic speakers. In certain applications, e.g. for driving bending wave panels or diaphragms as exemplified by WO98/39947, which are intended to be driven centrally, e.g. so that they can act both pistonically and in bending, such small diameter voice coils may cause power handling and excursion-related problems.
For such small diameter voice coils the drive point impedance (Zm) approximates to that of a panel driven at a single point. As the frequency is increased Zm oscillates with modal structure but is on average constant and approximates to the infinite panel value given by the following equation:Zm=8√{square root over (Bμ)}
As a result, for a given voice coil mass (Mc) there is a high frequency limit (f(b)) above which the rising impedance of this mass exceeds the constant drive point impedance. This frequency is given by the following equation:       f    ⁡          (      b      )        =            Z      ⁢                           ⁢      m              2      ⁢      π      ⁢                           ⁢      M      ⁢                           ⁢      c      
Consequently the voice coil mass on known bending wave panels has been kept low according to the above formula.
The obvious way is to increase Zm or reduce Mc in order to keep the turnover frequency high in the audio band. Voice coil diameter has only ever been increased slightly and then only to find that the cell cap, drum-mode resonance becomes dominant and causes premature roll-off.
Other issues that work against low mass voice coils for pistonically driven panels are sensitivity and bandwidth. In order to keep a realistic low frequency bandwidth in a realistic enclosed volume, the diaphragm mass needs to be high. So, to keep sensitivity up, the Bl force factor will need to be high. High Bl drivers usually rely on the number of turns to increase the Bl product and thus increase voice coil mass.
Another direction is to use an under-hung vibration exciter design relying on the magnet to increase the Bl product and thus keeping voice coil mass low. This has been tried using a 25 mm voice coil diameter and an increased stiffness over the drive point. But power handling and excursion are still restricted.
It is known from WO97/09842 to provide a flat panel loudspeaker which operates pistonically at low frequencies and which is resonant at high frequencies. It is also known from U.S. Pat. No. 4,542,383 to provide a loudspeaker having a moving coil transducer and a diaphragm, both being of similar diameter and the voice coil being arranged to drive the diaphragm around its periphery.